There are a number of situations in which an image of a scene that is moving with respect to the camera is required. One solution to this problem involves forming a number of sub-images in which each image is taken with the camera at a different position relative to the scene. The various sub-images typically overlap one another to some extent. The overlap is then used to combine the sub-images to produce the larger desired image.
Aerial photographic surveys typically utilize such a solution to generate an aerial view of a large geographic area that could not be photographed at the desired resolution in a single photograph. Typically, a camera is mounted on the underside of an aircraft that flies a predetermined pattern over the terrain to be photographed. Images are formed at predetermined intervals and combined later to provide the desired image.
The individual photographs must be taken under constraints imposed by the relative motion of the camera and the scene being photographed and by the lack of an auxiliary light source to improve the exposure. To prevent blurring of the image, the effective shutter speed must be very high. This constraint, in turn, limits the light that is available for any given exposure. The limited light cannot be augmented by an external light source such as a flash. Even in relatively bright daylight conditions, details in the shadows of trees or other objects may be lost due to the lack of light from these regions. Furthermore, the low light levels cannot be augmented by increasing the f-stop of the camera, since there is a minimum depth of field that is required for each photograph and that depth of field is determined by the variations in the terrain, not by the photographer.
In principle, the sensitivity of the camera can be increased by utilizing large lenses that collect more light. However, economic constraints limit this solution to the problem.
Another potential solution utilizes a scheme in which the overlap between the various images is increased so that each area on the final image is seen in a number of individual sub-images. The data for each pixel is then provided by combining the measurements from the corresponding sub-images, and hence, effectively increasing the exposure time.
However, there is a limit on the number of sub-images that can be taken. In digital photography, the image is projected onto an imaging array consisting of an array of individual light sensing elements that convert the light striking that element to an electric charge that is subsequently readout and digitized. The imaging array typically has several million sensing elements, and hence, the time to readout the imaging array into an associated memory for storage limits the number of frames that can be taken. In addition, the amount of high-speed memory needed to store all of the individual images increases the cost of the system. Finally, the substantial post photographic imaging processing needed to combine the various images to provide the desired composite image also presents substantial economic limitations on such solutions.
In addition, schemes in which the individual images are readout and digitized suffer from increased noise. In addition to the noise arising from the limited number of photons that are converted for each sub-image, each sub-image is also subjected to readout noise when the sub-image is readout and digitized.